1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, and more particularly to a control apparatus for an internal combustion engine having a plurality of cylinders, provided with a cylinder halting mechanism for halting operation of some of the cylinders.
2. Description of the Related Art
Japanese Patent Laid-Open No. Hei 8-105339 discloses a control system for an internal combustion engine provided with a cylinder halting mechanism. In this control system, partial-cylinder operation in which some of cylinders of the engine are halted, and all-cylinder operation in which all of the cylinders are operated, are switched according to an engine load, i.e., a throttle valve opening. Specifically, according to this control system, the partial-cylinder operation is performed when the throttle valve opening TPS is less than a switching throttle valve opening TPS1 which is set according to the engine rotational speed Ne.
If a vehicle driven by the internal combustion engine described above travels on highlands where the atmospheric pressure is low, the intake pressure of the engine is saturated with a comparatively small throttle valve opening. Accordingly, the output power of the engine does not increase even if the throttle valve opening is increased further. Therefore, according to the conventional control system described above, when the vehicle travels on high ground, the intake pressure is saturated before the throttle valve opening TPS reaches the switching throttle valve opening TPS1, even if the driver operates the accelerator pedal during the partial-cylinder operation to open the throttle valve. Accordingly, the engine output power does not increase any more. Thereafter, when the throttle valve opening TPS reaches the switching throttle valve opening TPS1, the all-cylinder operation starts (the partial-cylinder operation ends), and the engine output power increases. As a result, the engine output power does not linearly increase depending on the accelerator operation of the driver, which gives strange feeling to the driver.